The present invention relates to a vehicle brake system, particularly to a vehicle brake system employing a brake booster. The present invention also relates to a vehicle brake holding system capable of automatically maintaining the stoppage of a vehicle when it is stopped on a slope or the like and the driver of the vehicle intends to keep it stopped.
A conventional vehicle brake system, which can keep braking a vehicle through the use of a brake booster even if the depressive force applied to a brake pedal is removed has been disclosed in published Japanese Utility Model Application (OPI) No. 13854/85 (the term "OPI" as used herein means an "unexamined published application"). The vehicle brake system includes a power piston slidably supported in a shell. A valve mechanism is provided in a valve casing at the central portion of the power piston. A constant pressure chamber is defined in a forward position with regard to the direction of braking action of the power piston. A variable pressure chamber is defined in a rearward position with regard to that direction. A communication passage connects the constant pressure chamber and the variable pressure chamber to each other through the valve mechanism. A pressure passage connects the variable pressure chamber and a high pressure fluid source to each other through the valve mechanism. An input shaft puts the valve plunger of the valve mechanism into action to switch fluid passages to disconnect the constant pressure chamber and the variable pressure chamber from each other and feeds a high pressure fluid into the variable pressure chamber to move the power piston forward. A first solenoid valve opens or closes the communication passage between the valve mechanism and the constant pressure chamber. During the action of the brake booster, the communication passage is closed by the first solenoid valve so that the constant pressure chamber and the variable pressure chamber are disconnected from each other even if the communication passage is opened by the valve mechanism because the depressive force is removed from the input shaft. The depression of an accelerator pedal, the non-depression of a clutch pedal and so forth are detected for the conventional vehicle brake system to cease the continuance of the braking of the stopped vehicle.
In another conventional vehicle brake device disclosed in published Japanese patent application (OPI) No. 96036/84, the holding of the braking of a vehicle is gradually released, because a strong shock would be caused by the quick start of the vehicle if the holding of the braking were quickly released. This vehicle brake system does not employ a brake booster but functions so that a brake pedal is maintained in a depressed position by a solenoid. When the brake pedal is released from the depressed position, the supply of the braking force holding signal to the solenoid is not immediately halted but intermittently halted to gradually cease holding the braking of the stopped vehicle.
If the first solenoid valve of the first described conventional vehicle brake system is intermittently opened and closed and then completely opened, then similarly to the intermittent halting of the supply of the braking force holding signal to the solenoid in the second described conventional brake system, the vehicle can be smoothly started when the holding of the braking of the stopped vehicle is released. In that case, however, it is likely that the life of the first solenoid valve is shortened and chattering is caused by the intermittent opening and closing of the first solenoid valve. These are two significant problems.
When a vehicle is to be temporarily stopped with a standard brake system, a brake pedal is depressed to brake the vehicle through the use of a hydraulic brake. In the vehicular brake system, a brake booster is provided between the brake pedal and a hydraulic cylinder which functions to put the hydraulic brake into action. In the brake booster, atmospheric pressure and a negative pressure, which is produced in the intake manifold of an engine or produced by a vacuum pump, are introduced into a shell divided into two chambers. The difference between the atmospheric and the negative pressures is used to boost the depressive force on the brake pedal to put the hydraulic cylinder into action to provide power braking. However, it is an inconvenience for the driver of the vehicle to keep the vehicle stopped on a slope or the like by continuing to depress the brake pedal with the standard brake system. This is a further problem.
In order to solve the latter problem, the present application proposed an improved brake booster as disclosed in the U.S. patent application Ser. No. 53,994. In the improved brake booster, one passage connects a front and a rear pressure chamber which are defined in a shell by a diaphragmed piston in a front and a rear position with regard to the direction of action of the piston. Another passage connects to each other the rear pressure chamber and an external pressure fluid source. Then, a valve mechanism is regulated by an input shaft mechanically coupled to a brake pedal so that the two passages are switched. The improved brake booster also includes an output shaft which moves with the piston, which is displaced depending on the pressure difference between the front and the rear pressure chambers. The improved brake booster is characterized in that a solenoid valve for opening or closing the passage for connecting the front and the rear pressure chambers to each other is provided in the front pressure chamber and is attached to the piston.
When the improved brake booster is used in a vehicle brake system, operating conditions are detected by the sensors of a controller. When certain operating conditions are detected, the solenoid of the solenoid valve is energized to close the valve. At that time, the controller acts so that another solenoid valve provided in a passage connecting the rear pressure chamber to the atmosphere is opened for a prescribed time to introduce the atmospheric pressure into the rear pressure chamber to brake a vehicle by a braking force corresponding to the atmospheric pressure. The braking occurs even if the brake pedal is not depressed. When the held braking of the vehicle is ceased, the solenoid of the first solenoid valve is de-energized by the controller so as to open the passage connecting the front and the rear pressure chambers through the valve mechanism. The vehicle stoppage brake device has a problem that it is not provided with a fail-safe means against abnormalities such as the disconnection of wires for a vehicle speed signal and a clutch signal which carry some of the prescribed operating conditions necessary for the device.